Transmissions are used in vehicles to transfer torque from a drive unit to a vehicle load. For example, many transmissions transfer the torque from a vehicle engine to a vehicle load produced at the interface of the vehicle's wheels when a vehicle is being driven along a road. Typical transmissions include a gear train that is reconfigurable among a number of gear ratios to establish a particular rotational drive ratio between the vehicle engine and the vehicle load.
Some transmissions include hydraulic control systems configured to set the gear ratio of the gear train. Hydraulic control systems may include hydraulic circuits operated by a controller to engage and disengage clutches included in the circuit to set the gear ratio of the gear train. The hydraulic circuit is typically pressurized by a main pump that is mechanically driven by the vehicle engine when the vehicle engine is running. Sometimes, the hydraulic circuit will lose pressure when the main pump is not being driven and the hydraulic circuit may not be able to maintain a selected gear ratio when the vehicle engine is not running.